Early cardiac pacemaker conductors were composed of numerous fine, stranded stainless steel wires. Marked improvement in both fracture rate and flexibility resulted when stainless steel conductors were wound into small coils with a hollow core. The hollow core of the coils also improved implantation since a stylet could be passed through the core during implantation to stiffen the lead. Corrosion resistance was significantly increased when stainless steel was replaced with more corrosion-resistant platinum iridium and nickel alloys such as Co—Ni—Cr—Mo alloy, available commercially as MP35N®, from Standard Pressed Steel Co., Jenkinstown, Pa. Highly specialized conductors were formed from such alloys such as multifilar coiled conductors and drawn, brazed strand wire. The use of multiple filars avoids the loss of electrical continuity in the event that one filar breaks. Drawn, brazed strand wire provides a low electrical resistance in a wire with high fatigue strength. Multifilar coils can also be used in side-by side or coaxial arrangements with insulation separating the conductors to provide individual conductors for the transmission of separate signals or stimulation pulses.
One limitation of commercially available alloys suitable for medical lead conductors, such as MP35N or Co—Cr—Ni—Fe—Mo—Mn alloy (known as Elgiloy®, from Elgiloy, Ltd.), is that foreign inclusions of nitride, oxide and/or carbide bodies present in the alloy negatively influence the metal fatigue life. Inclusions differ in mechanical and physical properties from the bulk alloy matrix. Titanium (Ti) is deliberately added to the MP35N alloy melt and is a significant carbide/nitride former. The inventors of the present invention have found titanium-nitride inclusions at or near fracture initiation sites of MP35N alloy wires that were rotary beam fatigue tested. Specifically, relatively hard, cubic titanium-carbide and titanium-nitride inclusions in excess of one micron in cross-section located within approximately three microns of the wire surface have been found to promote fatigue crack initiation in cold drawn wires having diameters between approximately 0.005 and approximately 0.010 inches in diameter.
The formation of oxide, carbide and nitride inclusions is related to melt practices employed in producing an alloy and casting it into ingot forms. Elgiloy develops oxide-based inclusions during vacuum induction melting and secondary melting during electro-slag refining, which occur under ambient atmospheric conditions allowing light metal oxides to reach equilibrium conditions. Sub- to multi-micron diameter oxide inclusions result. Formation of titanium-based inclusions in MP35N is a process not fully understood but is expected to be related to pressure, temperature, elemental concentrations, and other equilibrium-driving factors present during alloy melt practices.
As patient indications for cardiac pacing expands, new pacing systems are being developed, such as multi-chamber or biventricular pacing systems, that require the use of relatively small diameter leads. These systems can use multiple leads, and multiple electrodes may be carried on a single lead requiring multiple conductors. In order to implant multiple leads through a venous access point, or advance a single lead through a narrow, tortuous pathway such as the cardiac veins, very small diameter leads are desired. Leads are presently being manufactured having a diameter on the order of 2 to 4 French. In order to manufacture such small diameter leads, conductor wires must be drawn very fine, on the order of 0.001 inch or less. As conductor diameter is reduced, the impact of inclusions on fracture resistance becomes greater. It is desirable, therefore, to provide a corrosive-resistant conductor having low electrical resistance that has improved fatigue resistance due minimization of the number and/or size of foreign inclusions.